Clades of γ-glutamyltransferases (GGTs) in the ascomycota and heterologous expression of Colletotrichum graminicola CgGGT1, a member of the pezizomycotina-only GGT clade

Gamma-glutamyltransferase (GGT, EC 2.3.2.2) cleaves the γ-glutamyl linkage in glutathione (GSH). Ascomycetes in either the Saccharomycotina or the Taphrinomycotina have one to three GGTs, whereas members of the Pezizomycotina have two to four GGTs. A Bayesian analysis indicates there are three well-supported main clades of GGTs in the Ascomycota. 1) A Saccharomycotina and a Taphrinomycotina-specific GGT sub-clade form a yeast main clade. This clade has the three relatively well-characterized fungal GGTs: (Saccharomyces cerevisiae CIS2 and Schizosaccharomyces pombe Ggt1 and Ggt2) and most of its members have all 14 of the highly conserved and critical amino acids that are found in GGTs in the other kingdoms. 2) In contrast, a main clade (GGT3) differs in 11 of the 14 highly conserved amino acids that are found in GGTs in the other kingdoms. All of the 44 Pezizomycotina analyzed have either one or two GGT3s. 3) There is a Pezizomycotina-only GGT clade that has two well-supported sub-clades (GGT1 and GGT2); this clade differs in only two of the 14 highly conserved amino acids found in GGTs in the other kingdoms. Because the Pezizomycotina GGTs differ in apparently critical amino acids from the cross-kingdom consensus, a putative GGT from Colletotrichum graminicola, a member of the Pezizomycotina, was cloned and the protein product was expressed as a secreted protein in Pichia pastoris. A GGT enzyme assay of the P. pastoris supernatant showed that the recombinant protein was active, thereby demonstrating that CgGGT1 is a bona fide GGT.